The present invention relates to a presensitized plate for use in making a lithographic printing plate (hereinafter referred to as "PS plate") and more specifically to a PS plate in which a coating layer of an organic polymeric compound is applied onto the back face thereof.
A positive-working PS plate which has been widely used comprises an aluminum plate serving as a support provided thereon with a photosensitive layer comprising an o-quinonediazide compound. It has been known that the o-quinonediazide compound is converted into a carboxylic acid through the irradiation with ultraviolet rays and accordingly if an exposed PS plate is developed with an aqueous alkaline solution, only the area on the photosensitive layer exposed to light is removed to expose the support surface. Since the surface of the aluminum support is hydrophilic, the surface (non-image area) of the support exposed through the development retains water and repels a printing ink. On the other hand, the area (image area) from which the photosensitive layer is not removed through the development is lipophilic. Therefore, the image area repels water and receives a printing ink.
On the other hand, a known negative-working PS plate comprises a substrate provided thereon with a photosensitive layer of a photosensitive diazonium salt, a photosensitive layer comprising a photopolymerizable monomer or a photo-crosslinkable photosensitive layer comprising cinnamic acid and a polymeric compound having dimethylmaleimido groups. The photosensitive layer is hardened through irradiation with light and only the unexposed area is removed by developing with a proper developer to likewise give a lithographic printing plate.
These PS plates have often been damaged when they are transported while putting a large number of these PS plates in layers.
Such defects are formed for the reason that a mat layer applied onto a photosensitive layer has insufficient slip characteristics with respect to the back face of an aluminum substrate and accordingly, the mat layer and the back face are rubbed together during transporting them in layers to thus damage the photosensitive layer. The mat layer is in general applied onto the photosensitive layer for reducing the time required for getting a vacuum during contact exposure to light in a vacuum printing frame and for preventing an image becoming indistinct. Moreover, when a large number of PS plates are stored while putting them in layers, it has sometimes been observed that the back face of the substrate is adhered to a photosensitive layer due to a load applied thereto and cannot be peeled off.
These problems have in general been solved by inserting an interleaf between neighbouring two PS plates put in layers.
However, if a large number of PS plates are used, it requires much labor to remove the interleaf and a great deal of interleaves are discharged as industrial waste. In particular, environmental protection has presently been advocated on a worldwide scale and, therefore, the amount of industrial waste should be reduced.
To eliminate this drawback, Japanese Patent Publication for Opposition Purpose (hereinafter referred to as "J. P. KOKOKU") No. Sho 51-6570 discloses a technique in which an easily peelable protective layer having a thickness ranging from 10 to 100 .mu. is applied onto the surface of a photosensitive layer or the back face of the substrate for PS plate. This technique makes it possible to eliminate the problem of scratches which are possibly formed during handling PS plates while putting them in layers, but cannot solve the problem of the adhesion of a photosensitive layer to a protective layer and further the protective layers peeled off have still been discharged as industrial waste.
Alternatively, Japanese Unexamined Patent Publication (hereinafter referred to as "J. P. KOKAI") No. Sho 50-151136 discloses a method comprising applying a layer of a soft polymer or paper onto the back face of a substrate; J. P. KOKAI No. Sho 57-63293 discloses a method comprising applying a protective layer of a polymer having a melting point of not more than 120 .degree. C. onto a composite substrate which comprises a surface-grained aluminum plate and a steel plate on the steel plate side (the side opposite to a photosensitive layer); J. P. KOKAI No. Sho 60-73538 discloses a method comprising applying a protective layer onto the back face of a substrate, the protective layer being peeled off prior to the development thereof; and J. P. KOKAI No. Sho 61-67863 discloses a PS plate provided with a layer of an organic polymer having a thickness of not more than 100 .mu. on the back face thereof. These prior art techniques are developed with emphasis on cutting the PS plates while putting them in layers and thus disclose that the preferred thickness of the protective layer should be not less than 100 .mu.m and that a relatively soft polymer is preferably used. In fact, a protective layer of a low density polyethylene having a thickness of not less than 10 .mu.m is favorable for cutting a plurality of PS plates put in piles. However, these PS plates are not designed in full consideration of the sliding of the photosensitive layer on the back face of the aluminum substrate and accordingly, the surface of the photosensitive layer is often damaged upon transporting these PS plates in piles.
Moreover, protective layers are swollen with an agent used during printing depending on materials used for forming the protective layers and the printing durability of the lithographic printing plate is deteriorated due to a change in the printing pressure. This problem is caused for the reason that the thickness of the protective layer is relatively thick in the order of not less than 10 .mu.m. J. P. KOKOKU No. Sho 55-48296 discloses a method comprising applying a layer capable of absorbing infrared light onto the back face of an aluminum substrate whose surface is mechanically, chemically or electrochemically roughened and provided thereon with a photosensitive layer and then heating (or burning) the plate from the back face. However, the IR-absorbing layer comprises, for instance, pigments (e.g., carbon black) or hard particles (e.g., fine metal particles) as an IR-absorber and thus the PS plate is not designed in full consideration of slip characteristics like the foregoing techniques. Therefore, the problem of scratches of the photosensitive layer cannot be solved and accordingly, the use of an interleaf is essential for these PS plates.
On the other hand, there has been known a PS plate comprising a substrate whose both surfaces are grained and provided with photosensitive layers on both sides of the substrate. It is likewise essential for these PS plates to use an interleaf for preventing the formation of scratches on the photosensitive layer.